Evaluation of quinidine effect on the antitumor activity of adriamycin and mitoxantrone in adriamycin-sensitive and -resistant P388 leukemia cells.

Utilizing the P388 murine leukemia cells sensitive (P388/S) and resistant (P388/ADR) to Adriamycin (ADR), we evaluated the effect of quinidine, an anti-arrhythmic agent, on the cytotoxic activity of ADR and Mitoxantrone (MITO), both in vitro as well as in vivo. Quinidine enhanced the cytotoxicity of both ADR and MITO in P388/S and P388/ADR cells, as assessed by the decrease in color intensity of formazan crystal in the MTT (3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide) assay. A dose dependent inhibition of 3H-thymidine and 3H-uridine incorporation was observed when the P388/S and P388/ADR cells were exposed to quinidine alone. A non-toxic concentration of quinidine (5 microM) enhanced the DNA biosynthesis inhibition induced by ADR (55 to 65%) and MITO (37 to 44%) in P388/ADR cells, indicating reversal of resistance, while in P388/S cells only a minimal increase in DNA biosynthesis inhibition was observed. The combination of quinidine at doses of 50 to 100 mg/kg significantly potentiated the antitumor activity of ADR and MITO in P388/ADR bearing mice, whereas the potentiation of ADR and MITO antitumor response was lower in P388/S bearing mice. Quinidine increased the cellular levels of ADR by 53 to 126% in P388/ADR cells in vitro, but failed to indicate such elevated levels of cellular ADR in P388/S cells. This enhanced intracellular accumulation of ADR in P388/ADR cells, explains the therapeutic efficacy of ADR and MITO in P388/ADR, both in vitro as well as in vivo. Results suggest the efficacy of quinidine to ameliorate the antitumor effects of ADR and MITO in drug resistant tumor cells.